Specifically the Onn 10.1" Gen 3, model 100071485? These are cheap, low-powered Walmart house brand tablets that are perfect for fucking around with, and I'm amazed there's not more of a modding community for them.
A year or so ago I screwed around with an older/smaller Onn Surf 8 (100071485), the punchline is ignore the awful android hacker crapware and use the open source MTKClient to talk to it. I haven't wholesale flashed it with a ROM or anything, but I have rooted and de-crapped and such.
There is a sad little subforum on xda for them, which at least is enough to see that it looks like the Gen3 ones are still Mediatek parts and the same approach should work.